The present invention relates to data communications and, more particularly, to novel methods of, and apparatus for, communicating digital data over a medium having also at least one potentially-interfering signal present therein.
Modern digital data communications must often be propagated over a medium in which at least one potentially-interfering signal is present. For example, the transmission of digital data by a modulated carrier over the A.C. power line is often impossible, due to the presence of potentially-interfering signals at each harmonic of the line frequency (e.g. 60 Hz. in the United States). The magnitude of modulated carrier signal is limited, by governmental regulation, to a relatively low value, which is typically less than the magnitude of the substantially-CW harmonics of the power line waveform, even at frequencies several orders of magnitude greater than the power line fundamental frequency. It has been proposed to use frequency-hopping (FH) spread-spectrum techniques to overcome the presence of a relatively few potentially-interfering signals; however, the multiplicity of relatively close-spaced harmonics of the power line frequency severely limits the rate at which digital data can be transmitted in a power line carrier (PLC) FH system. It has also been proposed to use frequency-shift-keyed (FSK) modulation of a single carrier, preferably precisely positioned between power line frequency harmonics, to allow an increase in data rate; however, the power line frequency harmonic "noise" severely limits the reliability of systems using an FSK modulation approach. Therefore, it is highly desirable to provide a method of, and apparatus for, communicating digital data over a medium, such as a power line, having a plurality of potentially-interferring signals, and providing greater reliability than an FSK-modulated system and a greater data transmission rate than an FH system.